It is sometimes advantageous to coat a substrate, especially a metal, with a coating having special properties, for example, wear resistance or corrosion resistance. Wear resistance can be provided for a substrate by coating it with a coating composed of a mixture of hard abrasive material such as powdered tungsten carbide or the like, dispersed in a hard filler metal. Corrosion resistance can be provided to a substrate with a similar coating composed of a corrosion resistant metal or alloy. Other special or improved physical properties can be obtained in a similar manner substituting other matrix materials.
Prior to the work of Breton et al. (U.S. Pat. Nos. 3,743,556, 3,916,506, and 4,194,040 the specifications and claims of which are incorporated herein by reference) it was difficult to produce such a coating, especially on objects having an intricate or complicated shape or requiring a coating thickness of greater than 0.020 of an inch. In the prior methods using plasma and flame spraying techniques, it was difficult to get uniform coatings on a substrate, especially a substrate having an intricate surface. A similar difficulty occurred in the prior method using techniques such as abrasive particle dusting onto the substrate.
Breton et al eliminated many of the difficulties that existed in the prior art methods by providing a method using a first layer of a desired thickness of a high melting point powdered matrix material in an organic binder material and a second layer of a lower melting temperature powdered brazing filler material also in an organic binder. The first layer is then placed on the substrate with the second layer in turn placed on top of it. The matrix material is characterized as being wetted by the brazing filler metal or alloy in the molten state. This assembly is then heated to decompose the binders and melt the filler metal or alloy which is infused by capillary action into the matrix layer. Cooling then yields an essentially void-free coating bonded on the substrate. The method for preparing both the hard particle matrix material as well as the braze filler alloy layers or preformed using fibrillated polytretra fluoroethylene (PTFE) is disclosed in U.S. Pat. Nos. 3,916,506 and 4,194,040. One of the big factors in determining ultimate physical properties by Breton et al method (wear resistance and/or corrosion resistance characteristics) of the coating is the amount of matrix particle packing. This matrix particle loading in the preform and the final coating layer is dependent in general on three material property factors: particle size, size distribution and shape. This has been found to be a limiting factor to this method. There has been a desire to try to increase this particle loading but the inherent properties of the powder material (particle size, size distribution and shape) and processing constraints have limited the success especially with articles in which the finish coating layer is in excess of 0.030 of an inch.